As an important function part, an automobile steering knuckle product needs to undergo strict dimensional control in a production process no matter for a blank or a finished product subjected to machining, and full-scale measurement using three coordinates is an essential link. According to field technological requirements, sampling inspection needs to be performed frequently, therefore, there is a need for a universal fixture used during three-coordinate measuring for realizing the loading and clamping for both the blank and the finished product subjected to machining. To prevent influences on the measurement, the fixture is strictly restricted in design, and the present invention solves this problem very skillfully and provides a fixture with a very simple structure.